First, just fixing a couple of typos (for the benefit of other helpers):
You multiplied by 4/4, not 2/2
"(2-4n)" should be (2-4k)

Next, a couple of observations:
What is the derivative of (1-x)m? What factor did you miss when you took the derivative before? If you're stumped by that question, I'll instead ask: what is the derivative of (1-x)?